This invention relates to a trouble diagnosis device which is able to detect whether or not an EGR (exhaust gas recirculation or return) system for controlling the EGR in the engine is out of order.
A trouble diagnosis device for EGR system has been well known in the art which electronically controls the operation of an EGR control valve provided in an EGR path through which part of the exhaust ga is recirculated being returned from an exhaust passageway to an intake passageway, thereby to control the amount of EGR. The EGR system controls an electromagnetic valve according to the speed and load of the engine (for instance an intake pipe pressure or a Q/N ratio where Q is the amount of air sucked, and N is the number of revolutions) for control of the amount of EGR.
A conventional EGR trouble diagnosis device of this type is as shown in FIG. 7. In FIG. 7, a conventional internal combustion engine 1 of 4-cycle spark ignition type is mounted on a motor vehicle. An intake pipe 3 is connected to the engine 1, thus forming an intake passageway for taking air through an air cleaner 2. The intake pipe 3 incorporates a throttle valve 4 in its upstream portion. The throttle valve 4 is to adjust the amount of air taken into the internal combustion engine 1. Furthermore, the intake pipe 3 has an electromagnetic injection valve 5 at a downstream position closed to the junction of the intake pipe 3 and the engine 1. The injection valve 5 is to jet fuel from a fuel system (not shown) into the engine. A pressure sensor 6 and a throttle sensor 7 are provided downstream of the throttle valve 4. The pressure sensor 6 operates to detect the absolute pressure in the intake pipe 3 and to convert it into a voltage signal. The throttle sensor 7 operates to detect the degree of opening of the throttle valve 4, to output a voltage corresponding to the degree of opening thus detected. The outputs of the pressure sensor 6 and the throttle sensor 7 together with the outputs of a crank angle sensor (not shown) and a cooling water temperature sensor (not shown) are applied to an electronic control section 8. The electronic control section 8, with those outputs as input data, controls the electromagnetic injection valve 5, and controls an electromagnetic valve 9, thereby to control the EGR (exhaust gas recirculation).
An EGR control device comprises: an exhaust branch pipe 11 which is connected to an exhaust pipe 10 to receive part of the exhaust gas from an exhaust passageway formed by the exhaust pipe 10; an EGR control valve 12 provided in an EGR passageway formed by the exhaust branch pipe 11; and an EGR inlet pipe 13 through which the downstream portion of the EGR control valve 12 is communicated with an intake passageway in the intake pipe 3 which is located downstream of the throttle valve 4. The EGR control valve 12 comprises: a negative pressure chamber 12A; a valve body 12B of diaphragm type which is opened by the negative pressure in the negative pressure chamber 12A; and a spring 12C urging the valve body 12B to close the control valve. The negative pressure chamber 12A of the EGR control valve 12 is connected through the electromagnetic valve 9 to a control negative pressure pipe 14 which is connected to the intake passageway at a position close to the throttle valve 4 downstream of the latter 4. The electromagnetic valve 9 has an air open valve 15. That is, the electromagnetic valve 9 is so designed that, when it is at a close control position, the air open valve 15 is closed to couple the negative pressure chamber 12A to the control negative pressure pipe 14, and when it is at an open control position, the negative pressure chamber 12A is opened to the air. When the negative pressure chamber 12A of the EGR control valved 12 is coupled to the control negative pressure pipe 14 by means of the electromagnetic valve 9 set to the close control position, the opening degree of the valve body 12 is changed by the negative pressure in the negative pressure chamber 12A, so that the amount of exhaust gas returned from the exhaust pipe 10 through the valve body 12B and the EGR inlet pipe 13 into the internal combustion engine 1 is controlled.
In the electronic control device for EGR system thus organized, the electronic control section 8 utilizes the outputs of the pressure sensor 6, the throttle sensor 7, the crank angle sensor, and the cooling water temperature sensor as input data, in which the control section 8 controls the operation of the electromagnetic injection valve 5 to inject the most suitable amount of fuel for purification of exhaust gas, and detects a control zone for EGR, and controls the operation of the electromagnetic valve 9 so that the operation of the internal combustion engine may not be adversely affected, thereby to control the operation of the EGR control valve 12. That is, when the EGR operation is required, the electronic control section 8 outputs a control signal to close the electro-magnetic valve 9 which has been opened to the air, as a result of which the negative pressure chamber 12A of the EGR control valve 12 is coupled to the control negative pressure pipe 14. The negative pressure downstream of the throttle valve 4 is applied to the negative pressure chamber 12A, so that the balance of the negative pressure with the elastic force of the spring 12C of the EGR control valve 12 determines the amount of lift of the valve body 12B, and the exhaust gas is returned at a flow rate corresponding to the amount of lift thus determined. When the EGR operation is not required, the electronic control section 8 outputs a control signal to set the electromagnetic valve 9 to the closed position. As a result, the negative pressure chamber 12A is opened through the air open valve 15 to the air, and the valve body 12B is fully closed, thus stopping the recirculation of the exhaust gas.
The conventional EGR system is designed as described above. This system has only simple means for detecting faults with high accuracy such that the EGR control system becomes out of order or unsatisfactory in operation, or that the valves and/or pipes are clogged up by contamination to make it impossible to recirculate a predetermined amount of exhaust gas with the result that the exhaust gas becomes worse in quality.